Hi, Mrs. D--
Not sure, but you might be seeing the results of low blood pressure. My hubby has Parkinson's disease, which can cause the Autonomic Nervous System to dysfunction (the part of your brain/nervous system that controls breathing, heart rate, blood pressure, etc.). He has many of these "dizzy spells."
A good idea is to get a blood pressure cuff so you can check his blood pressure when these episodes happen. I've found the easiest to use are the "wrist cuff" type. You can get them, at least in the US, for less than $50. They are REALLY easy to use--just put on the wrist & push a button. The reading will readout in about a minute. The newer models have a memory function, so you'll have a record of the date/time & BP and heart rate reading to share with your doctor.
There are medications, pacemakers, and other alternatives the doctor can offer to help overcome this, if it is indeed a heartrate or blood pressure problem. Sometimes, it's just a simple issue like dehydration. Unfortunately, with Alzheimer's patients, they can't really tell you if they're thirsty or not. Sometimes, like their sense of smell, these functions don't "work" for them any more like they do for the rest of us, so they don't even realize they need some water or food.
I've found it's important to keep hubby hydrated (plenty of water, spaced through the day). When he gets dehydrated, the low blood pressure can be a problem (and sometimes it will even go too high, due to his disease). Cardiologist is aware of it all & guiding us.
We all know how scary these things can be & how powerless you can feel. But it does make you feel a little better when you can understand a bit more about what's going on (like seeing a blood pressure reading that's too low or too high). Doctor will help you, too, once they see the numbers. Hope this helps you; good luck!